The present invention relates to the fabrication of substrates such as semiconductor wafers, and more specifically, to fluid-based processes such as immersion lithography for patterning one or more layers of the semiconductor substrate.
Semiconductor device geometries have dramatically decreased in size since such devices were first introduced several decades ago. Since then, integrated circuits have generally followed the two year/half-size rule (often called Moore's Law), which means that the number of devices on a chip doubles every two years. Today's fabrication plants are routinely producing devices having 0.13 micron and even 90 nm feature sizes.
Due to the ever shrinking feature sizes, changes have been made throughout the semiconductor manufacturing process. For example, lithography is a mechanism by which a pattern on a mask is projected onto a substrate such as a semiconductor wafer. In areas such as semiconductor photolithography, it has become necessary to create images on the semiconductor wafer which incorporate minimum feature sizes under a resolution limit. Lithographic systems must use shorter light wavelengths to form the smaller features. One solution has been a process called immersion lithography. Immersion lithography uses a transparent fluid to fill the space between a projection lens of a scanning or step-and-repeat lithography system and the substrate (e.g., semiconductor wafer) surface.
For further example, in a 193-nm wavelength exposure system, it is common to use water as the fluid between the projection lens and the substrate surface. This works well because the lens can be designed with numerical apertures higher than one, which allows the lithography system to produce smaller images and thereby shrink the feature sizes.
There are a number of practical issues to implementing immersion lithography. For one, maintaining a consistent bubble-free fluid between the lens and the wafer surface is very difficult. There are basically three approaches to the problem. The first approach is to submerge the entire wafer and lens in a pool of water. The issue with this approach is that a complex system of servo motors and laser interferometers are required to accurately move the chuck, and submerging some or all of this system is difficult to achieve. The second approach is to limit the pool size to the top of the chuck. This technique would keep all of the chuck control mechanisms out of the water but would add considerable mass to the chuck that must rapidly accelerate. The third technique is to dispense the water between the lens and the wafer with a nozzle and rely on surface tension to maintain a “puddle”. However, bubbles can still form between the lens and the wafer surface due to the fact that water droplets can be created all over the wafer surface, and not just at the puddle. When an unexposed portion of the wafer that includes a water droplet receives the puddle, air can be trapped, thereby causing one or more bubbles.
It is desired to provide a method for use with fabrication processes such as immersion lithography that reduces or otherwise eliminates any bubbles that may occur between on the wafer surface.